The invention relates generally to an osteosynthetic anchoring member, and in particular to a fixation device for fixating fractured femoral heads including such an anchoring member.
In the osteosynthetic treatment of fractures of the femoral neck, i.e. fractures of the femur in which the femoral neck linking the condyle with the rest of the femur is broken off, the bone fragments are generally temporarily linked together by means of a fixation device. One type of fixation device used for this type of fixation includes a) an anchor bolt to be screwed into a fractured condyle which has a headless shaft and an internal screw thread on its rear end; b) a plate to be screwed to the main part of the femur with a sleeve for receiving the shaft of the anchor bolt, and c) a compression bone screw to be screwed into the internal screw thread formed on the rear end of the anchor bolt, the head of which is supported by the rear shoulder surface of the sleeve. One example of such a device is shown in Switzerland Patent Application No. CH 634,742 to Sutter. A disadvantage of this type of anchor bolt resides in the fact that on screwing in the compression bone screw, a rotation of the anchor bolt must be avoided, as any turning of the anchor bolt would cause the anchor bolt to be driven further into the condyle instead of the desired effect of pressing the bone fragments against each other. For this reason, the anchor bolt in the sleeve must be prevented from rotation by means of a prismatic shape of the anchor bolt shaft and of the bore formed in the sleeve or by means of a key-and-slot connection between the anchor bolt and the sleeve. Thus, on implanting the fixation device, the surgeon is faced with the problem of having to place the plate with the sleeve in such a way over the anchor bolt that it engages exactly with the anti-rotation means. This difficulty is particularly aggravated by the fact that typically the anchor bolt does not protrude from the bone but is sunk into it by about 10 mm, which means that the surgeon cannot see the anchor bolt and has to spend much time trying to find the adequate position of the sleeve.
A need exists for an improved osteosynthetic anchoring member, and in particular for an osteosynthetic anchoring member which permits the sleeve to be placed in a simple manner over the anchoring member after the latter has been inserted into the bone, and which comprises an anti-rotation means arranged between the sleeve and the anchoring member to avoid a rotation of the femoral head, and which in addition optimally absorbs the occurring physiological strains. It is also desirable that the anchoring member allow an optimization of the strain absorption without necessitating a greater dimensioning of the anchoring member.
The present invention relates to an osteosynthetic anchoring member comprising a longitudinal shaft extending along a longitudinal axis having a front shaft portion and a rear shaft portion, the front shaft portion insertable into a bone and comprising anchoring means for fixating the shaft in the bone and the rear shaft portion comprising anti-rotation means for permitting the rear shaft portion to be received in an implantable bone plate or a connecting member and to be secured against rotating about the longitudinal axis. The anchoring means are capable of being extended orthogonally with respect to the longitudinal axis of the shaft, and the rear shaft portion comprises tension means for extending the anchoring means to enable the anchoring member to be fixated within the bone.
In one embodiment, the osteosynthetic anchoring member according to the invention comprises a prismatic or cylindrical shaft with a diameter D, a resilient anchoring means situated within said shaft and capable of bending outward radially relative to said shaft, and tension means located on the rear end of said shaft by means of which the anchoring means may be reversibly bent outward so as to enable the anchoring member to be anchored within the bone. The anchoring member has a longitudinal axis, a front end insertable into a bone, and a rear end insertable into an internal plate or a connecting member. By means of the tension means, the anchoring means may be bent outward in an arch-shaped manner so as to achieve a diameter Dmax greater than D. The ratio Dmax:D may be between 1.2 and 3, preferably between 1.5 and 2.5. The outward bending of the anchoring means advantageously takes place over a length L, which is between 10 and 60 mm. Preferably, the anchoring means may be bent outward elastically. In other embodiments, however, the anchoring means may also be subject to plastic deformation.
In one embodiment of the osteosynthetic anchoring member according to the invention, the anchoring means are shaped in the form of anchoring wires with a diameter d, each of which having a rear end adjacent or facing the tension means and a front end located opposite with respect to the longitudinal direction of the shaft. Preferably, in one embodiment, the anchoring member is equipped with between 3 and 6 anchoring wires.
The anchoring wires may be arranged parallel to the longitudinal axis, both ends thereof being located within the shaft and the anchoring wires being apt to be bent outward vertically to the longitudinal axis in an arch-shaped manner as the tension means is actuated. The diameter d of the anchoring wires is preferably between 0.5 mm and 2.5 mm, preferably between 1 mm and 1.5 mm. Advantageously, the ends of the anchoring wires are preferably spherical or ball shaped, the diameter of the balls being preferably greater than the diameter d. In another embodiment of the osteosynthetic anchoring member, the tension means is shaped in the form of a spindle which is arranged coaxially to the longitudinal axis of the shaft and the external screw thread of which may be screwed into a corresponding internal screw thread of a threaded sleeve which is located within a bore formed in the shaft in such a way as to extend concentrically to the longitudinal axis and to be secured against axial displacement and rotation. The spindle comprises a bearing member with an annular groove located towards the front shaft portion and displaceable within the bore parallel to the longitudinal axis. The annular groove is arranged on the bearing member in a cross-section vertical to the longitudinal axis and serves for receiving the rear ends of the anchoring wires. The bearing member is connected to the spindle in such a way that it is secured against axial displacement but capable of rotating about the longitudinal axis. In the lateral area of the bore formed in the shaft, a groove is arranged which extends parallel to the longitudinal axis and engages with a finger projecting radially from the bearing member. The configuration of the groove in the bore of the shaft and of the finger on the bearing member allows said bearing member to be axially displaced by the spindle while being secured against rotation relative to the shaft. Thus, it can be avoided that the turning of the spindle results in a torque exerted on the anchoring wires. For the purpose of rotating the spindle, means for receiving a screw driver, such as a hexagon socket or a groove, may be provided on the rear end thereof. In addition, the spindle may be provided on its rear end with a coaxial bore including an internal screw thread for receiving a compression bone screw.
In another embodiment, a plug member is positioned on the front end of the shaft and partially inserted into the bore and which may be convex, preferably spherical on the side opposite to the front shaft end, the convex part of the plug member forming the front end portion of the anchoring member. On its portion projecting into the bore, the plug member may in turn be provided with an annular groove, extending in a plane vertical to the longitudinal axis, which serves for receiving the front ends of the anchoring wires. Advantageously, both annular grooves have a circular cross-section. The rear shaft end may be provided with means for receiving a screw driver, shaped for example in the form of a groove. With the aid of a screw driver blocking said groove, the shaft may thus be secured against rotating together with the spindle, as the spindle is turned.
The invention also relates to a fixation device that serves for fixating fractured femoral heads and comprises in addition to the anchoring member according to the invention, which is fixed within the spongiosa of the fractured condyle by an outward bending of the anchoring wires, a plate to be screwed against the main part of the femur including a sleeve in which the part of the anchoring member adjoining its rear end may be received and wherein said anchoring member is displaceable coaxially to the longitudinal axis, and a compression bone screw to be screwed into the internal screw thread formed in the spindle on the rear shaft end, the head of which is supported by a shoulder surface of the sleeve. By rotating this compression bone screw, the fractured femoral head may be pulled close to the neck of the femur. An anti-rotation means is provided between the anchoring member and the sleeve, preventing a rotation of the femoral head about the longitudinal axis of the anchoring member.
Advantageously the aptitude of bending outward radially of the anchoring means of the osteosynthetic anchoring member allows the anchoring member to be anchored within a great volume of the bone. This may be of particular advantage in cases of osteoporotic bone. In addition, the plate with the sleeve may easily be placed over the anchoring member of the present invention, once said anchoring member has been anchored in the bone, which greatly facilitates the implantation of the entire fixation device.